I found a great website (the URL escapes me now, but Google "bedwetting") that explains why kids wet the bed. Studies have been conducted which prove what parents have been saying all along, "My child is a very sound sleeper, that is why they don't wake up to go potty." They put headphones on kids, who then went to sleep. The researchers sent loud sounds through the headphones, and the kids slept right through them.
Also, your body has to start producing a hormone called anti-diruretic hormone. Some kids start producing ADH right when they achieve daytime control. Most kids take a lot longer. Doctors aren't even interested until the child is 6 years old and still wetting at night. Unfortunately, prescribing ADH only has a 20-30% success rate, and doctors don't like prescribing hormones at such a young age.
That said, my daughter was daytime trained at 2 1//2. She achieved nighttime dryness at 4 1/2 when her ADH finally kicked in. We used pull-ups (which she started keeping dry in August)until last November when she asked to wear her big girl panties to bed. She has had only two accidents since then, one because she couldn't get out of bed fast enough because of her bed rail. Than was a blessing in disguise because it convinced her to give up her bed rail. I keep a little potty in her bedroom with a very low wattage night-light nearby so she can find it, and she can get herself up at night to use it.
Starting last August, I would take her potty before going to bed, around 11:00 pm (some nights it was tough staying up that late) and my husband would take her at 5:00 am when he got up. In November, I started getting up at 2:00 am to take her potty. I could tell the ADH was starting because she wouldn't produce a lot of urine. This March, I decided she could make it through the whole night, and she has. She even gets up herself when she needs to go, and goes back to bed. The ADH has really kicked in, sometimes its 45 minutes before she has to relieve herself after she gets up in the morning.
I restrict her fluids after dinner (unless she's been running about and is obviously thirsty),but she can drink what she needs during the day. When i was a kid, I think the method was to keep kids slightly dehydrated. I swear my mom wouldn't let me drink more than 12 oz of fluids a day, even during the summer with no air conditioning.
Some will say never go back to the pull-ups, but if you and your son are frustrated every morning with the bedwetting, I say use the pull-ups at night (honestly, they didn't confuse my daughter. She knew to use the potty during the day). Anything that helps ease even a bit of frustration is worth it. If anyone says you shouldn't use the pull-ups, invite them to come over and change your son's bed and wash the bedclothes everyday. They'll change their tune. .
Hope this helps. He'll get it eventually.